I think the best way would be getting one from a store as supergeekgirl said.

But if you can't find anything that works, maybe try using a ping pong ball, cutting it so it fits your forehead nicely, then you can cut the iris hole and put something like this painting the iris and pupil on the back; then cover it all with glossy varnish, or even clear nail polish will work (if you want to paint it with a light orangey pink and then add some nerves it wold look even better). Then use scarwax, or latex for the eyelid, add fake lashes or make up or anything else you want to make it look realistic. And voilą!

I did it for an eye necklace (sorry I don't have any photos) and it looked pretty realistic, and it costed practically nothing because If you look around and ask, probably you will find everything you need.